The present invention relates to an apparatus and a method for the thermal treatment of substrates, especially semiconductor wafers, including at least one heating device for heating at least one substrate by means of electromagnetic radiation, whereby the heating device includes at least two arc lamps.
An apparatus of the aforementioned type is known, for example, from U.S. Pat. No. 4,698,486, according to which a semiconductor wafer that is located in a reaction chamber is initially heated by an arrangement of quartz halogen lamps. After the wafer has reached a certain temperature, an additional lamp arrangement, namely a high-power pulsed lamp arrangement, is used for heating the wafer. The pulsed lamp arrangement is used only for a short period of time, whereby the surface temperature of the wafer is raised to a temperature in the vicinity of the melting point of the semiconductor material. Due to the short periods of use of the pulsed lamps, the temperature of the wafer is influenced by the pulsed lamps only in the region of the wafer surface, which leads to a non-homogeneous temperature distribution over the thickness of the wafer. The quartz halogen lamps, as well as the pulsed lamps, are disposed in a highly reflective chamber that surrounds the reaction chamber and that is built up in the form of a kaleidoscope. In order to achieve a homogeneous temperature distribution on the surface of the wafer in the reaction chamber, it is important with this known apparatus that the spacing between the respective lamps and the surface of the wafer be greater than or at least equal to the diameter of the reflective chamber. The spacing between the respective lamps and the wafer surface, in particular with an apparatus that is suitable for heating both sides of the wafer, leads to a large overall size of the apparatus. With the ever increasing wafer diameters, such as for example 300 mm wafers, it is therefore necessary to have a very large reflective chamber, which leads to high manufacturing and maintenance costs of the apparatus.
From U.S. Pat. No. 4,398,094 as well as U.S. Pat. No. 5,336,641 respective apparatus are known for the thermal treatment of semiconductor wafers, according to which in each case an individual arc lamp having a mirror is used as the heat source. The arc lamps used as the heat source are thereby generally high-power arc lamps having an expensive construction and a complicated cooling device. Due to the use of an individual lamp, it is not possible to achieve a homogeneous thermal treatment of the wafer.
From DD-A-295950 it is furthermore known to use UV radiating systems having varying configurations in combination with glow lamps for the thermal treatment of semiconductor wafers. In this connection, a metallic vapor, low-pressure UV source is used that contributes only insignificantly to the overall radiation density of the heating device. The object of the UV radiation system is not the actual heating of the wafer, but rather the promotion of photochemical reactions in conjunction with thermally activated processes via the UV irradiation.
Most of the presently commercially available apparatus for the thermal treatment of substrates utilize exclusively glow lamps for the thermal treatment of semiconductor wafers. However, these lamps have the drawback that the radiation of the glow lamps is only very slightly absorbed at wafer temperatures below 600xc2x0 C. This is due to the characteristic spectrum of the glow lamps, which at wave lengths of about 1000 nm is at its maximum. The rate of absorption of an Si semiconductor substrate is, however, for wave lengths in this range greatly dependent upon temperature and varies from about 0.1 to 0.7. Only at temperatures of more than 600xc2x0 C. is the rate of absorption in this wave length range nearly independent of wave length. The result of this is that the energy of the glow lamps can be effectively absorbed only at a wafer temperature of greater than about 600xc2x0 C.
Proceeding from the known apparatus, it is an object of the present invention to provide an apparatus of the aforementioned type that enables an effective economical and homogeneous thermal treatment of substrates.
Pursuant to the present invention, this object is realized with an apparatus of the aforementioned type in that the radiation characteristics for each arc lamp can be individually controlled by a control device, and in that the electromagnetic radiation of the arc lamps contribute substantially to the power density of the electromagnetic radiation of the heating device. The use of arc lamps, which contribute significantly to the power density of the electromagnetic radiation of the heating device, has the advantage that these lamps radiate in a spectral range in which, for example Si wafers, have the highest absorption, and in particular already at room temperature. Thus, as a result of the arc lamps an effective heating in particular of semiconductor wafers is possible even at room temperatures. Due to the possibility of an individual control of the arc lamps, the spatial radiation field of the lamps can be established, which enables a homogeneous radiation distribution and thus a homogeneous temperature distribution upon the wafer surface. Due to the ability to set the spatial radiation field, the heating device, including the arc lamps, can be disposed in the vicinity of the substrate that is to be treated, and it is not necessary to maintain a certain spacing between the heating device and the substrate, which spacing is prescribed by the diameter of the treatment apparatus, in order to be able to carry out a homogeneous substrate treatment. As a result, the overall size of the apparatus, and the cost connected therewith, can be considerably reduced.
For an optimal adaptation of the radiation characteristics of the apparatus to the process conditions, especially to the substrate that is to be treated, the operating mode and/or the lamp current of the arc lamps is advantageously individually controllable. The arc lamps are advantageously controllable in a direct current operation and/or in a pulsed manner as flash or pulsed lamps. For an effective use of the arc lamps, these contribute at least {fraction (1/10)} to the power density of the heating device. For a good, effective cooling of the arc lamps, these are preferably fluid cooled. As a result of a good cooling of the arc lamps, their life expectancy can be extended considerably.
In order to achieve a good, homogeneous temperature distribution on the substrate that is to be treated, the gas or glow discharge zone of the arc lamps corresponds essentially to a dimension of the substrate, such as the edge, length or the diameter of the substrate. It is preferably longer than the dimension of the substrate. The arc lamps are preferably disposed in the region of the outer periphery of the substrate in order to be able to easily control the temperature distribution. Arc lamps are particularly suitable in this region, because they have a rapid response characteristic.
The heating device preferably has a bank of lamps having at least two rod-shaped lamps that are disposed nearly parallel to one another. In this connection, the lamps of the bank of lamps have, preferably in addition to the arc lamps, glow lamps that enable an economical heating of the substrates. Pursuant to one specific embodiment of the invention, the glow lamps and the arc lamps have essentially the same dimensions so that they can be interchanged in order to be able to specifically adapt the radiation characteristics within the apparatus to the respective substrate or the respective thermal process. The glow lamps and the arc lamps advantageously have essentially the same lamp power, so that the same cooling system can be utilized for the lamps. For an increased effectiveness of the apparatus, the heating device is surrounded by a chamber that at least in stages reflects the electromagnetic radiation, so that radiation of the lamps that is not directed directly upon the substrate is reflected upon the substrate. In this connection, the spectral reflection coefficient of the chamber is preferably dependent upon position in order to achieve a certain spectral radiation distribution upon the substrate surface. In particular, it is possible to prevent the UV radiation of the arc lamps from being reflected upon certain regions of the substrate that do not face the arc lamps.
In order to reduce the overall size of the apparatus, the spacing of the heating device relative to the surface of the substrate is preferably less than the diameter of a reaction chamber. In this connection, the ratio between spacing and diameter is advantageously less than 0.5.
Pursuant to a particularly preferred specific embodiment of the invention, the radiation characteristics of the electromagnetic radiation of the heating device can be modulated, as a result of which a good possibility of determining the temperature of the wafer is provided. In this connection, it is an advantage of the arc lamps that they can be operated with considerably higher modulation frequencies than is true for halogen lamps. As a result, the temperature of the object, especially during the temperature fluctuations, can be determined more precisely and with simpler evaluation electronics.
The heating device and/or the substrate is advantageously disposed in a nearly homogeneous, adjustable magnetic field, the lines of flux of which, at least in the vicinity of the arc lamp anode have a component that extends essentially parallel to the arc discharge of the arc lamps. A modulation of the lamps can be effected by the magnetic field. Furthermore, by means of the magnetic field a significant influence can be had upon the life expectancy of the arc lamps, since the rate of erosion of the anode can be positively reduced. In this connection, the magnetic flux density of the magnetic field is advantageously between 0.005 and 0.3 tesla.
For a thermal treatment on both sides of the substrate, the apparatus preferably has a second heating device, whereby the substrate is disposed between the heating devices.
The apparatus advantageously has a reaction chamber that essentially surrounds the substrate and is nearly transparent for the electromagnetic radiation of the heating device, the reaction chamber advantageously being provided with quartz glass and/or sapphire. The reaction chamber material advantageously has an absorption coefficient, averaged over the spectrum of the electromagnetic radiation, between 0.001 per centimeter and 0.1 per centimeter. The thickness of the wall of the reaction chamber is preferably between one millimeter and five millimeters.
Pursuant to one preferred specific embodiment of the present invention, the glow lamps at least partially have a helical filament structure with which preferably a predefined geometrical and spectral radiation profile can be achieved. For this purpose, the filament preferably alternatingly has helical and non-helical filament structures. Pursuant to a further, preferred specific embodiment of the invention, at least one glow lamp has at least two individually controllable filaments. Preferably at least one filament of a glow lamp has at least three electrical connections. The density of at least one filament structure preferably varies along the filament.
Pursuant to a preferred specific embodiment of the invention, the reaction chamber and/or the lamp bodies form a frequency filter for at least one wave length range of the electromagnetic radiation of the heating device in order within this spectrum to determine the radiation reflected from the wafer and hence to determine the temperature, whereby the lamp radiation is suppressed by the filter effect. Such a frequency filter can be achieved, for example, in that synthetic quartz is selected for the lamp bulbs and fused quartz is selected for the reaction chamber. The filter effect of such a frequency filter can be additionally influenced by the selection of the temperature of the lamp body. In particular at low temperatures the intrinsic emission of the lamp body is additionally reduced. As a result, approximately at 2700 nm the lamp radiation is suppressed, the wafer radiation of 2700 nm can, however, be detected through the reaction chamber. In general, such frequency filters can be built up as absorption filters or in the form of interference filters, in other words, by utilizing thin dielectric layers. These advantages can also be advantageously achieved in that at least one of the lamps is at least partially filled with a material that absorbs a specific wave length range of the electromagnetic radiation of the heating device. With the halogen lamps, a suitable additive can, for example, be mixed with the halogen gas of the lamp filling, with such additive absorbing in narrow bands and advantageously emitting only slightly or not at all in the absorption band.
The object of the invention can be realized with a method of the aforementioned type in that the radiation characteristics for each arc lamp are individually controlled, and the electromagnetic radiation of the arc lamps contributes substantially to the power density of the electromagnetic radiation of the heating device. This results in the advantages already mentioned above.
Pursuant to a particularly preferred specific embodiment of the invention, the substrate is heated in a lower temperature range essentially by means of the arc lamps, since arc lamps give off radiation in a spectral range that, for example, by SI semiconductor wafers, is absorbed better at temperatures below 600xc2x0 C. than is the radiation of glow lamps. The substrate is preferably heated in a lower temperature range exclusively by means of arc lamps. For Si semiconductor wafers, the lower temperature range is preferably between room temperature and approximately 600xc2x0 C.
The electromagnetic radiation of the heating device is preferably modulated, which pursuant to one specific embodiment of the invention is effected by applying a magnetic field in the region of the substrate and/or the heating device, with the lines of flux of the magnetic field, at least in the region of the arc lamp anode, having a component that extends essentially parallel to the arc discharge.
The invention will be explained in greater detail subsequently with the aid of a preferred specific embodiment and with reference to the drawings; the drawings show: